This present invention is directed to an apparatus for use in the productivity of optical fiber and more particularly, to a lathe collet chuck for use in the MCVD process.
In the manufacture of optical fibers, a process known as MCVD (modified chemical vapor deposition) which is shown and described in U.S. Pat. No. 4,217,227 of J. B. MacChesney et al., which is incorporated herein by reference, is widely used and has become an industry standard for the production of glass pre-form tubes with the requisite chemicals deposited on the interior walls thereof. An initial step of the process involves injecting into an axially revolving glass tube mounted in a lathe chuck, which is referred to as a starter tube, a gaseous mixture containing certain reactants while the tube is being heated by a moving torch. The particle components of the gaseous mixture, i.e., the soot, are deposited on the interior walls of the tube in the region thereof of greatest heat, and, since the torch moves longitudinally, the soot is deposited along the length of the tube. After sufficient soot is deposited, the starter tube is heated and collapsed into a glass preform rod from which the optical fiber is subsequently drawn. During the deposition process, the remnant gases and reactants after the deposition process exhaust out of the end of the tube remote from the gas introduction end. It is not uncommon for a buildup of soot to occur at the exhaust end of the tube to the point where free flow of the mixture out of the tube is inhibited, which can have a deleterious effect on the deposition process. As a consequence, it is the usual practice to butt fuse an exhaust tube to the exhaust end of the starter tube, the exhaust tube having a larger internal diameter than the starter tube to permit free flow of the exhaust gas mixture from the starter tube into and through the exhaust tube. In order that the butt fusing produces a smooth joint, the butting ends of one or both tubes are shaped, such as a flare on the starter tube and/or a chamfer or taper on the exhaust tube prior to the deposition process. In U.S. patent application Ser. No. 09/562,476 of Mueller et al., filed Aug. 31, 2000, the disclosure of which is incorporated herein by reference, there is shown an apparatus for imparting the desired shape to the butting end of the two tubes, which involves mounting, gripping, and rotating the tubes being shaped in a lathe chuck.
Commonly, each tube is supported in the headstock shuck of common lathe bed, or in the headstock chuck and a tailstock chuck. The most widely accepted method of mounting a tube is to use a scroll chuck with the jaws of the chuck clamping the outside diameter of the tube, with the tube being cantilevered and having a free distal end. Other arrangements use collet chucks or other swaging devices to develop the clamping forces. In most, if not all, prior art arrangements, the generation of the clamping force is determined by the operator establishing a preload or through the use of springs or other force generating members.
In greater detail, existing systems utilize, for example, a three-jawed scroll chuck to provide the clamping. The chuck jaws and other components thereof are reasonably stout enough to be considered rigid bodies. Thus, when the clamping force is applied by rotation of the scroll, the glass starter tube (or exhaust tube) may be deformed slightly to have the resultant system retain clamping loads. Glass is a poor spring, hence any slight change in the scroll torque or temperature related expansion tends to relieve the clamping load very quickly. Some operators interpose a soft material between the chuck jaws and the glass tube to act as a buffer for the uneven loading and thermal expansion effects. The material and the clamping technique used have assumed the status of an xe2x80x9cartxe2x80x9d in achieving repeated and accurate results, where slipping tubes or cracked tubes are the penalty for improper clamping. Some degree of accuracy and repeatability can be achieved with the use of pneumatic chucks which involve an assembly of intricate movement linkages and piston drivers, all connected through complex rotary unions for the transmission of the clamping force energy. It is also possible to utilize geared preloaded chucks and electromagnetic clamping devices, with the concomitant problem of a complex arrangement of components. In all such cases, the complexity dictates a regimen of periodic maintenance and of even more often adjustment.
The present invention includes a compliant collet chuck that achieves repeatable and proper clamping forces and that is of simple construction and preforms the delicate gripping adjustment without requiring any particular skills of the operator.
In greater detail, the chuck of the invention, in a preferred embodiment thereof, comprises a collet base plate which is attachable to the spindle of the lathe and from which extends an upstanding portion. The base has a central bore surrounded by a channel, and has a tapered section or cone for receiving the adjustable collet chuck which has a rearend tapered portion that rides in the tapered cone of the base. There are, in this embodiment, three such collets that are spaced from each other and spring loaded, by springs in the spaces, with respect to each other. The collets are held together in a circular configuration having a central opening for the work piece by a circular spring member which surrounds them. With such a collet structure, as the collet assembly is moved toward the base member, the taper in the base bears against the taper at the rear of the collets and forces them radially inward, thereby applying a clamping force to a work piece in the central opening or bore.
A bayonet cylinder rides in the channel surrounding the tapered central bore of the base and has, at its inner or base end three circumferentially spaced circumferential bayonet slots into which radially disposed eccentric pins fit. The other end of the bayonet cylinder has internal threads for receiving an axially free play unit. The nut has external threads for mating with the threads of the cylinder, and a taper bore therein which bears against the tapered front portion of the three collets. Thus, as the nut is screwed into the cylinder, the collets are moved toward the base and the collets are moved radially inward by the taper in the base of the taper in the nut.
As pointed out hereinbefore, the end of the bayonet cylinder that rides in the channel, i.e., the proximal end, has three circumferentially spaced circumferential bayonet slots therein and the base has three radially disposed eccentric pins designed to fit within the slots in the bayonet locking arrangement. The eccentric pins, which, as will be seen hereinafter, function as cams, are equally spaced about the circumference of the bayonet cylinder and are mounted in the base, and extend into and through the channel perpendicularly in a single plane relative to the cylinder axis of rotation, which coincides with the axes of the tapered bore in the base and the tapered bore in the nut. Each cam is a shaft assembly having an eccentric section and having an exterior handle for rotating the cam. The camming surface is an eccentric surface ground smaller than the shaft diameter and tangent to the shaft diameter at a single point. Each of the bayonet slots in the bayonet cylinder has an arcuate seat for the camming portion of each pin, and the remainder of the slot, which is too narrow to accept the pin, defines a circumferential beam between the slot and the end of the cylinder. Thus, when the cylinder is inserted in the channel, each pin, upon rotation of the cylinder, comes to rest in its respective arcuate seat. Rotation of the pin causes camming pressure on the beam which acts to pull the cylinder further toward the base member, thereby adding an increment of tightening of the collets to the glass tube. The beam has a certain resilience which allows the arrangement to accommodate temperature changes in dimension of the glass tube.
In operation, the glass tube is mounted in the chuck and the nut is tightened to where the collets bear against the tube and to where any slack is removed. The eccentric or cam pins are then rotated to cam the bayonet cylinder down which causes the collets to grip the tube sufficiently for subsequent operations. The resilience of the beam allows for slight variations in tube diameter, whether as made or by thermal expansion.
In most instances, it is desirable that there be no rotation of the collets relative to the base member. This can be assured by means of an axial groove on one or more of the collets, and a mating pin in either the base or the bayonet cylinder. Such an arrangement allows axial movement of the collets but prohibits rotation thereof.